Solid-state laser sources operated at deep UV wavelength around 200 nm are greatly desirable for photo-refractive surgeries. Such a deep UV laser source is expected to be more compact, more reliable, and require less maintenance in comparison with the excimer lasers, which is currently the dominant laser source for photo-refractive surgeries. More importantly, solid-state laser sources can be operated at a much higher repetition rate and with much less energy fluctuation compared with the excimer lasers. Scanning a deep UV laser beam with high repetition rate enables a variety of ablation shapes on a cornea surface and provides a great flexibility for the refractive surgeries. The improved stability in pulse energy from a solid-state UV laser source ensures accurate and controllable ablation.
In U.S. Pat. No. 6,031,854 to Lai, a diode pumped cascade laser is proposed for deep UV generation. The second laser employs a short cavity with only a gain medium and a wavelength selection element inside the cavity. When pumped by a laser pulse of 50 ns or shorter, the second laser is gain-switched to produce a pulse of nanosecond duration. This nanosecond laser pulse is then converted to deep UV radiation by a wavelength converter.
In the above approach, a short pump pulse is critical for generating a single short pulse with nanosecond duration and millijole energy. The pulse build-up time is proportional to the laser cavity length and inversely proportional to the net pump pulse energy above the lasing threshold of the cavity. When the pump pulse duration is longer than the build-up time of the laser pulse, a second pulse will appear. This results in smaller energy in the first pulse and thus lowers the conversion efficiency in deep UV generation.
It is well known in the art that a master oscillator—power amplifier system is a common approach to obtain amplified pulses of short duration, good beam profile, and narrow bandwidth. In such a system, the master oscillator is usually a low gain, low power laser to produce a seed pulse of certain specifications. The power amplifier is a high gain, high power laser to amplify the seed pulse up to much higher pulse energy. A number of master oscillator—power amplifier systems are commercially available from, for example, Lambda Physics of Germany and Continuum of Santa Clara, Calif.
The advantage of a master oscillator—power amplifier system is that the oscillator and the amplifier laser cavities can be optimized independently. The system, however, requires two pump sources and two gain media. Also, the system requires additional optics to inject the seed pulse from the oscillator to the amplifier and to isolate the amplified pulse from feeding back to the oscillator. As a result, a master oscillator—power amplifier system is usually complicated and expensive.